A frequently recurring problem in communications is the need to remove noise and interference from a signal. There have been many approaches suggested to address this problem. For stationary narrowband interference, a simple notch filter may be effective. Adaptive notch filters have been suggested to remove narrowband non-stationary interference. For removal of broadband noise from speech, Wiener filter techniques such as spectral subtraction are frequently used.
An adaptive filter can be used to remove a single non-stationary narrowband interfering signal from a broadband, or speech, signal. In this process, the frequency of the interfering signal is estimated at each time, and a single estimate of the interfering signal bandwidth is estimated. The received signal is frequency shifted at each time such that the frequency of the interfering signal is constant in the shifted representation. A fixed notch filter is applied to remove the interfering signal, and the filtered signal is frequency shifted to its original frequency. This process can remove a single interfering narrowband signal and it can, in principle, be iterated to remove several interfering signals. However, the primary problem with using an adaptive filter is that the instantaneous frequency of the interference must be accurately estimated at each time. Such estimation is difficult. Furthermore, an adaptive filter cannot be used to remove noise.
In Wiener filtering, or spectral subtraction, components which are dominated by noise are removed from the spectrogram, and the noise-free, or clean, signal is estimated by a pseudo inversion process. Spectral subtraction is the most common enhancement method used in speech processing. In this method, a short time Fourier transform (STFT) is computed. The STFT is a time-frequency representation. STFT components identified as dominated by noise are reduced in magnitude, and a signal whose STFT approximates the modified STFT is computed. The clean signal estimated this way is only an approximate solution because the modified STFT cannot be inverted. In the pseudo inversion process, some ad hoc criterion must be used. Furthermore, spectral subtraction cannot be easily used to remove narrowband interference.
For stationary narrowband interference, a clean signal can be estimated by using a notch filter which is tuned to the interference frequency and bandwidth. A notch filter cannot remove noise or interference whose frequencies change with time.
None of the prior art methods adequately identify and separate signal and non-stationary interference components.
U.S. Pat. No. 6,175,602, entitled “SIGNAL NOISE REDUCTION BY SPECTRAL SUBTRACTION USING LINEAR CONVOLUTION AND CASUAL FILTERING,” discloses a method of reducing noise in a signal by spectral subtraction. The present invention does not use spectral subtraction as does U.S. Pat. No. 6,175,602. U.S. Pat. No. 6,175,602 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 6,266,633, entitled “NOISE SUPPRESSION AND CHANNEL EQUALIZATION PREPROCESSOR FOR SPEECH AND SPEAKER RECOGNIZERS: METHOD AND APPARATUS,” discloses a device for and method of noise suppression that uses blind deconvolution. The present invention does not use blind deconvolution as does U.S. Pat. No. 6,266,633. U.S. Pat. No. 6,266,633 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 6,795,559, entitled “IMPULSE NOISE REDUCER DETECTING IMPULSE NOISE FROM AN AUDIO SIGNAL,” discloses a method of reducing noise by detecting and smoothing the high frequency amplitude of a signal, attenuating the non-smoothed amplitude of the signal, comparing the attenuated amplitude to a threshold, and identifying and removing the noise. The present invention does not smooth the high frequency amplitude of a signal as does U.S. Pat. No. 6,795,559. U.S. Pat. No. 6,795,559 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 6,801,889, entitled “TIME-DOMAIN NOISE SUPPRESSION,” discloses a method of reducing noise by performing a Fourier Transform on the signal to generate a frequency spectrum, performing an Inverse Fourier Transform to simulate a noise signal, and subtracting the simulated noise signal from the time-domain signal. The present invention does not simulate noise using an Inverse Fourier Transform as does. U.S. Pat. No. 6,801,889. U.S. Pat. No. 6,801,889 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 6,826,392, entitled “MULTIPATH NOISE REDUCTION METHOD, MULTIPATH NOISE REDUCER, AND FM RECEIVER,” discloses a method of reducing noise by frequency modulating the signal, extracting a high-frequency
signal from the modulated signal, generating a noise reduction coefficient from the extracted high-frequency signal, separating the signal into high-frequency and low frequency components, multiplying the high-frequency component by the noise reduction coefficient, and adding the product to the low-frequency component. The present invention does not generate a noise reduction coefficient as does U.S. Pat. No. 6,826,392. U.S. Pat. No. 6,826,392 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. Appl. No. 20020173276 A1, entitled “METHOD FOR SUPPRESSING SPURIOUS NOISE IN A SIGNAL HELD,” discloses a method of reducing noise by determining a distribution function of the signal, comparing the distribution to a reference distribution, modifying the components in the signal that differ from the reference distribution, and not modifying the components in the signal that are the same as those in the reference distribution. The present invention does not use a reference distribution as does U.S. Pat. Appl. No. 20020173276 A1. U.S. Pat. Appl. No. 20020173276 A1 is hereby incorporated by reference into the specification of the present invention.